Marshmallow Masher

Explore the powerful properties of air as you put tasty treats to the pressure test.

Marshmallows are a delicious, fluffy staple of summer, campouts, and barbecues. Did you know that there isn’t really much to them? It’s true. The best way to see what really comprises a marshmallow is to put it to the Marshmallow Masher pressure test. You’ll use the power of air to demonstrate what you’re really eating when you roast ‘mallows around the campfire.

Experiment Materials

Safety Notes: You should always practice safe science practices when performing experiments and demonstrations, and the Marshmallow Masher is no exception.

  • Always wear safety glasses.
  • Never over-pressurize (> 40 pumps) any container.
  • Always release the pressure before storing the bottle.


  1. Fill a 16 oz plastic bottle 3/4-full with miniature marshmallows.
  2. Attach a Fizz-Keeper Pump to the 16 oz bottle by screwing the pump onto the mouth of the bottle.
  3. Begin pumping air into the bottle with the Fizz-Keeper by pressing and releasing the pump. You will notice that air is forced into the bottle, but not released. The Fizz-Keeper uses a valve that only allows air to flow in one direction: into the bottle.
  4. While pumping air into the bottle (no more than 40 pumps!) watch what happens to the marshmallows! As more air is forced into the bottle, the marshmallows begin to wrinkle and shrink. The more pumps you give the Fizz-Keeper, the smaller the marshmallows become.
  5. Once you’ve mashed your marshmallows enough, carefully unscrew the Fizz-Keeper, but don’t take your eyes off of the marshmallows. When the pressure is released from the bottle, the marshmallows rapidly expand into their original size and shape!How Does It Work?

How Does It Work?

The Fizz-Keeper pump is like a miniature bicycle pump that fits snugly onto the mouth of plastic bottles. Pumping the Fizz-Keeper forces molecules of air into bottles, just like inflating a tire. While marshmallows are present in the bottle, this increased air pressure makes the marshmallows shriveled, wrinkly, and shrunken. Why? Well, it’s because marshmallows are just sugar and air. In fact, marshmallows are comprised of about 40-60% air. When you increase the air pressure inside of the bottle beyond the regular atmospheric pressure outside, the force actually crushes the marshmallow’s air pockets.

You can use this demonstration to discuss the effects of atmospheric pressure. We sometimes refer to things as being “light as air,” but the truth is that the air surrounding our planet weighs a lot and exerts considerable pressure on us. The atmospheric pressure at sea level is 14.7 pounds per square inch of surface area. That’s roughly the weight of 2 gallons of milk resting on 1 square inch!

A typical marshmallow has a surface area of about 6 square inches. So, the marshmallow has about 88 pounds of atmospheric pressure being exerted upon it (6 square inches x 14.7 pounds per square inch = 88.2 pounds). The marshmallow is full of tiny bubbles of air. The air pressure inside these tiny bubbles is roughly the same as the air pressure pushing on the marshmallow from the outside, so the pressures are equalized and the marshmallow retains its regular shape

Additional Info

What would happen if you pulled all of the air out of the bottle instead of pumping extra molecules into the bottle? Take a look at the Incredible Growing Marshmallow Trick.